IV HIRUDINEA 157 



cord fused together, while at the hinder end a group of seven gangha 

 are similarly fused. Sensory cells are scattered through the epidermis ; 

 here and there these are clumped together in definite sensory papillae 

 which are seen regularly arranged on the outer surface of the body and 

 at the front end five pairs of these on the dorsal surface of the following 

 annuli — i, 2, 3, 5, 8 — are converted into eyes. These can be clearly 

 seen as black spots it the front end of a pale-coloured specimen is held 

 close to a lamp so that the light shines through it. 



The Hirudinea as a group are annelids which have become specialized 

 in adaptation to their bloodsucking semiparasitic habits. They are 

 characterized by their suckers — the anterior one for fixing the lips round 

 the incision which they make in the skin of their prey, the posterior 

 for adhering. The development of the posterior sucker at the hinder 

 end, where during the growth of an ordinary annelid new segments are 

 added to the body, serves to bring this increase in the number of somites 

 to an abrupt stop after a more or less definite number (not exceeding 34) 

 has been reached. 



The somites themselves have become far less distinct than they are 

 in the other annelids — in external view owing to the absence of chaetae 

 and parapodia and to the superficial subdivision of most of the somites 

 into annuli, in internal structure owing to the spacious coelomic ca%ity 

 with its division into distinct compartments having become converted 

 into a continuous spongework. A characteristic difference in detail 

 is that in the Leeches the original paired genital openings have been 

 shifted into the middle line so as to become unpaired. 



Included in the group Hirudinea are a number of different genera. 

 One of these — Acanihobdella, found attached to fresh-water fishes in 

 the great lakes of Northern Russia — is of great interest from the evolu- 

 tionary point of view, being as it is a link which serves to connect up 

 the Leeches with the other annelids, for it possesses on its first five 

 somites perfectly typical chaetae and its coelome is still a spacious body- 

 cavity divided up by about twenty incomplete transverse septa. 



Amongst the Leeches more closely allied to the medicinal Leech 

 {Hirudo medicinalis) are the genera Aulostoma, the Horse Leech, which 

 commonly leaves the water to devour earthworms — its favourite food, 

 Haemadipsa, the unpleasant Land Leech of the tropics, and Nephelis 

 one of our commonest small fresh-water Leeches, which feeds on worms 

 and molluscs. 



Another set of Leeches are characterized by the saws having dis- 

 appeared and a protrusible proboscis having taken their place. Amongst 

 these are Pontobdella — a large marine leech which attacks Skates and 



